There is proposed, as a conventional technology, an ultrasonic sensor that is attached to a bumper of a vehicle for detecting obstacles around the vehicle. (Refer to Patent Document 1)                Patent Document 1: JP-H04-35600 A        
Here, a vibrating planar member (or vibrating face) of this type of the sensor is exposed in its surface outward of the vehicle, so this exposed surface is painted in the same color as the bumper of the vehicle. Further, the ultrasonic sensor is attached to the bumper or regions around the bumper, so thrown stones or gravels often hit the sensor to cause dents on the surface of the vibrating planar member. These dents or damages then bring about corrosion on the surface.